Lifetime progesterone (P4) exposure is an important contributor to breast cancer risk, as reflected in the risk associated with the lifetime number of ovulatory cycles, increased breast epithelial proliferation accompanying the P4 peak in the luteal phase, and the higher breast cancer risk of exogenous progestin users. Moreover, the tumors seen with progestin use appear to be more aggressive. This clinical and epidemiological evidence points to progesterone blockade as an excellent candidate strategy for breast cancer prevention. A new generation of selective progesterone receptor modulators (SPRMs) is now available; one of these (telapristone acetate, TPA) inhibits P4-driven proliferation of breast cancer cells, and attenuates P4-driven tumor growth in carcinogen treated rats. We have identified a novel set of genes associated with proliferation of breast cancer cells in response to the progestin, R5020 that are effectively suppressed by TPA. Based on these data as well as findings from others, we hypothesize that P4 promotes a pro-proliferative and tumor permissive phenotype which supports breast cancer development, suppression of which by SPRMs will significantly decrease breast cancer risk. The breast-specific effects and mechanisms for potential breast cancer protection of SPRMs are unknown. We propose to define the mechanisms by which SPRMs antagonize PR at the molecular level, and the role of coactivators and corepressors. In addition, the efficacy of SPRMs in antagonizing the proliferative response to P4 will be determined, to guide selection of patients for SPRM therapy. In Aim 1, we will determine the mechanisms by which SPRMs regulate PR activity in breast cells. The ability of SPRMs to affect binding to the PR will be studied; we will perform ChIP-Seq to determine how SPRMs affect PR recruitment to the genome in a global manner, and the involvement of the nuclear receptor corepressors, NCOR and SMRT as well as other transcription cofactors will be assessed. In Aim 2, we will determine if SPRMs inhibit the P4-mediated tumor permissive phenotype; we have defined a 16 gene panel, which is associated with P4 driven proliferation. We will test this in vitro, in human mammary organoids, to determine whether SPRMs inhibit expression of these signature genes. We will also study the ability of SPRMs to inhibit mammary stem cell expansion associated with P4 exposure, and the proliferation and growth of tumors that carry BRCA1 mutations. In Aim 3, we will evaluate the expression P4-response genes in human clinical samples, a) in high and low ambient progesterone environments and b) following treatment with telapristone acetate. These experiments will 1) define modes of actions of SPRMs in cells and tissues where PR signaling is active, 2) relate inhibition of proliferation of breast cells, and of stem cell expansion, to speciic genetic pathways; and 3) demonstrate utility of these markers in human breast samples. These results will significantly advance the field of breast cancer prevention in novel directions, providing both new, effective agents, particularly for premenopausal women.